The Raman effect was first discovered by Sir C. V. Raman in 1928. He observed that when a substance (solid, liquid, or gas) is irradiated by a monochromatic light whose frequency does not correspond to any of the absorption lines of the substance, frequency shifted components can be detected in the scattered radiation. These shifted components, or Raman lines, have shifts independent of the irradiation frequency but characteristic of the material itself. The shifted lines on the low frequency side were also mirrored by shifted lines on the high frequency side, although the latter were less intense.
These phenomena describe what is commonly termed spontaneous Raman scattering. This process involves an inelastic scattering mechanism in which a photon incident on an atom or molecule causes it to undergo a change in internal energy. In the case of an atom, this internal energy is in the form of an electronic transition. However, the scatterer can be a molecule in which case the change in internal energy is in the form of a vibrational and/or rotational transition. The scattered photon is thus shifted by the exact change in internal energy. If a molecule is originally in the ground vibrational state, Raman scattering will shift the scattered photon to longer wavelengths at an energy E=h(.nu..sub.o -.nu..sub.vib), where .nu..sub.o is the frequency of the incident photon (also called the pump photon)and .nu..sub.vib is the frequency of the vibrational transition. If the scattering molecule is originally in a vibrationally excited state, the scattered photon may be shifted to shorter wavelengths at an energy E=h(.nu..sub.o +.nu..sub.vib). Photons which are downshifted in energy, leaving the scattering molecule in a higher vibrational state, are called Stokes photons. Photons which are upshifted by the Raman process are conversely called anti-Stokes photons.
Stimulated Raman scattering (SRS) is the stimulated analog to the spontaneous Raman effect. This occurs when the presence of the Stokes photons stimulates the interaction of pump photons with the Raman active media creating more Stokes photons. Classically, the stimulated Raman process can be thought of as the constructive interference between incident radiation and that at a Stokes shift which further drives material oscillations. This effectively creates an exponential gain for the scattered Stokes wave at the cost of the incident pump wave.
The stimulated Raman process is characterized by an intensity threshold above which gain at various Stokes frequencies .nu..sub.o -.nu..sub.vib can be induced. Usually only the vibrational transition having the largest spontaneous Raman intensity per line width is "active" in a particular molecule. The stimulated scattering process is a coherent scattering process in which, provided the input intensity exceeds threshold, a significant number of pump photons are scattered into various Stokes components.
A simple method for achieving the necessary threshold intensities for SRS is by using a laser as the pump and focusing the beam through the scattering medium. In this case, the gain near the focus can be high enough that spontaneous Raman scattering provides sufficient input for the stimulated process. The resulting stimulated emission is along the forward and backward directions of the incident beam, since these have the highest gain-length product. When the laser bandwidth is larger than the spontaneous Raman line width of the molecules (or atoms), the stimulated Raman emission takes place mostly in the forward direction. Energy conversions of incident pump to Stokes shifted output of greater than 50% have been observed.
One application of stimulated Raman scattering, especially in the forward direction, lies in this ability to efficiently convert radiation at one wavelength to another wavelength. Thus, for example, high efficiency rare gas-halide lasers, with output in the ultraviolet, have been shifted into other spectral regions via SRS.
Since the discovery of rare-gas halide lasers in 1975, considerable effort has been expended in using the stimulated Raman scattering (SRS) process in conjunction with these efficient, high power devices to achieve broad ultraviolet and visible wavelength coverage.